As a p-type ohmic electrode of a presently widely available nitride semiconductor light-emitting device, various metal thin films, such as Ni/Au, Pt/Ni/Au, Ni/Pt/Au, Ti/Pt/Au, or Pd/Au, have been thoroughly studied. In particular, an Ni/Au thin film is mainly applied to the p-type electrode of the GaN semiconductor light-emitting device.
However, an ohmic contact of such a metal thin film results in light being absorbed by the semi-transparent metal electrode, thus causing many problems, such as decreased luminous efficiency, thermal stability, and reliability of the device. Moreover, since the p-type GaN thin film has high resistance, when the size of the light-emitting device is increased, current is not uniformly transferred to such a device. Therefore, it is difficult to fabricate a large light-emitting device having high luminance.
To solve the problems, a method of fabricating a device comprising a thin film type transparent electrode is disclosed in the literature “Indium tin oxide contacts to gallium nitride optoelectric devices” (Applied Physics Letters. 74, 3930 (1999)). However, the contact resistance of the light-emitting device increases, and thus, the efficiency of the device decreases.
Although an organic electroluminescent (EL) device is available in addition to the semiconductor light-emitting device, it is also disadvantageous because light emitted from the light-emitting layer does not escape the electrode due to the refractive index (varying with the kinds of material) according to Snell's law, and may be reflected within the light-emitting device or may be absorbed by the metal electrode, thus decreasing the luminous efficiency.